Cyclic ester oligomers (CEOs) have been known for a long time; see for instance U.S. Pat. No. 2,020,298. They are known to be present in varying, usually small, quantities in many linear polyesters and have been isolated from such linear polyesters; see for example A. G. Harrison, “Analysis of cyclic oligomers of poly(ethylene terephthalate) by liquid chromatography/mass spectrometry”, Polymer, 38(10), 2549–2555 (1997) and G. Wick, H. Zeitler, “Cyclic Oligomers in polyesters from diols and aromatic dicarboxylic acids”, Angewandte Makromolekulare Chemie, (1983), 112, 59–94. They are often low viscosity liquids, and it has long been known that they may be polymerized to higher molecular weight linear polyesters by ring opening polymerization; see for instance U.S. Pat. Nos. 5,466,744 and 5,661,214 and references cited therein. This ability to readily form a high molecular weight polymer from a relatively low viscosity liquid has made these CEOs attractive as materials for reaction injection molding type processes, wherein a low viscosity material is converted to a high molecular weight polymer in a mold, so that a final shaped part is obtained.
However such CEOs have been difficult and expensive to prepare, for example requiring very high dilution conditions and/or using relatively expensive starting materials such as diacyl halides in conjunction with diols and a base to react with the HCl formed; see for instance U.S. Pat. No. 5,466,744. These high manufacturing costs have in many cases prevented the use of CEOs commercially, and therefore lower cost routes to CEOs are of great interest.
More recently it has been found that polyesters can be made from dicarboxylic acids or their diesters and diols using enzymes that catalyze (trans)esterification; see for instance X. Y. Wu, et al., Journal of Industrial Microbiology and Biotechnology, vol. 20, p. 328–332 (1998), E. M. Anderson, et al.; Biocatalysis and Biotransformation, vol. 16, p. 181–204 (1998); and H. G. Park, et al., Biocatalysis, vol. 11, p. 263–271 (1994). In some instances, in such reactions the production of small amounts of CEO coproducts has also been reported; see for instance G. Mezoul, et al., Polymer Bulletin, vol. 36, p. 541–548 (1996). There has also been a study reported on the amounts of CEOs that should be present in such reactions; see C. Berkane, et al., Macromolecules, vol. 30, p. 7729–7734 (1997). The latter study concluded that formation of the CEOs in the enzyme catalyzed reactions followed the same type of rules that govern the formation of these CEOs in nonenzymatic catalyzed reactions, and that only small fractions of CEOs should be produced in such enzymatic reactions unless they were done under very dilute conditions. In the processes described in all of these references the byproduct alcohol or water from the transesterification/esterification was removed (usually by sparging with an inert gas) to drive the polymeric product to higher molecular weight.
A recent paper, A. Lavalette, et al., Biomacromolecules, vol. 3, p. 225–228 (2002), describes a process whereby an enzymatically catalyzed reaction of dimethyl terephthalate and diethylene glycol or bis(2-hydroxyethyl)thioether leads to essentially complete formation of the dimeric cyclic ester, while use of 1,5-pentanediol leads to a relatively high yield of the dimeric cyclic ester, along with some linear polyester. The formation of high yields of the cyclic ester with diethylene glycol and bis(2-hydroxyethyl)thioether is attributed to a π-stacking-type short range interaction which favored formation of the dimeric cyclic ester.
Heretofore, however, it has been unknown in the art how to produce CEOs from the reaction of dicarboxylic acids with diols in a continuous process to obtain the CEOs in an amount that is greater than that predicted by thermodynamic equilibrium, as taught by H. Jacobson and W. H. Stockmeyer in “Intermolecular Reaction and Polycondensation I. The Theory of Linear Systems”, The Journal of Chemical Physics, Vol. 18 Number 12, December 1950, and which is well-known to persons skilled in the art.
Surprisingly, it has been found that when linear ester oligomers (LEO'S) are reacted in the presence of an esterification/transesterification enzyme catalyst in a non-aqueous medium using the continuous process of the present invention, significant quantities of cyclic ester oligomer can be obtained.